Device and method for switching optical data for optical communication networks

ABSTRACT

The field of the invention is that of switching optical signals with carrier wavelength conversion capacity, comprising a set of input ports (PE 1 -PEn), a set of output ports (PS 1 -PSn) functionally connected to the input ports so that an input signal presented to one of the input ports may be selectively routed to at least one of the output ports, and wavelength conversion means ( 34 ) providing a capacity for converting an input signal carrier wavelength to at least one other output port output wavelength.  
     According to the invention the wavelength conversion capacity of said conversion means ( 34 ) is limited by at least one of the following three limitation means i) to iii): i) for at least one of said output ports (PS), no wavelength conversion may be applied for sending a signal from an input port; ii) for at least one of said output ports (PS), wavelength conversion may be applied for sending a signal from an input port (PE), but to only a restricted number of wavelength values from the number L of different wavelength values accepted at the input, this restricted number being greater than 0 and less than L, and iii) for only a restricted number of output ports (PS) less than the total number of output ports of the switching system, wavelength conversion may be applied for sending a signal from an input port (PE) to any wavelength value from the number L of different wavelength values accepted at the input.

The invention relates to the transmission of data in opticalcommunications networks and more particularly to architectures for theswitching matrices that are provided at nodes of the network toswitching data to different links and peripherals of the network. Thesematrices must be able to convert the wavelength λ of the switched datacarrier as and when required, in particular to multiplex the data and/orto solve contention problems that arise when transporting data on thesame fiber. The data may take the form of time division multiplexpackets, wavelength division multiplex packets or a circuit mode datastream that is continuous from end to end.

The function of a switching matrix is to gather and forward data bymeans of a set of input ports and a set of output ports, respectively.Between the input and output ports, the data may be subjected to carrierwavelength conversion or to a predetermined time-delay to ensure itscontinued safe passage through the network. To this end, switchingmatrices are equipped with:

-   -   multiplexers for converging different separate data channels        onto the same fiber,    -   if the matrix manages wavelength division multiplexed data,        demultiplexers to carry out the converse process of extracting        channels from a set of channels multiplexed onto respective        fibers,    -   wavelength converters, which generally have the additional        function of amplifying an optical signal, and    -   where applicable, delay lines that act as buffers for separating        streams of data in time, in particular to solve contention        problems.

With the ongoing growth of data traffic on optical networks, it isbecoming necessary to cater for higher and higher bit rates in opticaldata switching matrices, which imposes complex node architectures thatare costly because of the volume of hardware that this implies.

The structure of optical packet switching matrices typically relies ontotal flexibility of carrier wavelength conversion at all input ports;thanks to wavelength converters associated with the output ports, eachinput signal may therefore be sent to any output port on any carrierwavelength accepted by the network.

This approach based on total wavelength conversion capacity makes thearchitectures particularly complex, since it implies multiplyingresources in order to ensure that each input wavelength can be forwardedfrom each output port on all available wavelengths. This leads to highoptical losses and to the need for a large number of components, evenfor architectures that operate in the wavelength domain, i.e. that usewavelength division multiplexing (WDM), precisely with the object ofreducing the number of active components.

FIG. 1 is a diagram illustrating the theory of an optical datacommunications network 2 comprising a set of nodes, of which some nodes4 operate only within the network and other nodes 4′ connected to thenodes 4 are located at the periphery of the network to set up externalconnections, for example with a gateway. Each node 4 or 4′ comprises oneor more switching matrices whose ports are connected to links 6 internalto the network 2 and also to network connection lines 8 in the case ofthe peripheral nodes 4′. The links 6 and connection lines 8 consist ofoptical fibers which, in the example considered here, use wavelengthdivision multiplexing to convey a plurality of data packetssimultaneously on one fiber.

FIG. 2 is a simplified representation of one example of a conventionalarchitecture for a switching matrix 10 of a node 4 or 4′ of the network2. In this example, the matrix is an n×n matrix, i.e. it has a number nof input ports PE1 to PEn (generic designation PE) and the same number nof output ports PS1 to PSn (generic designation PS).

Each input port is connected to a respective input optical fiber cableFE1 to FEn and each output port is connected to an output optical fibercable FS1 to FSn. Each of the input and output optical fiber cables isable to convey a number L of different carrier wavelengths λ1 to λL, therespective ports being adapted to multiplex and/or demultiplex all Lwavelengths.

There is a cross-connection unit 12 between the input ports PE and theoutput ports PS so that each wavelength of each input port may beconnected to all wavelengths of all n output ports, as symbolized by thecrossing lines within this unit.

A time-delay unit 14 is provided for imposing a selected time-delay onthe lines connecting the input and output ports of the cross-connectionunit 12. These time-delays are used in particular to regulate contentionwhen a plurality of data channels wish to access the same output port onthe same wavelength at the same time.

FIG. 3 shows in more detail a portion of the FIG. 2 matrix at the levelof the input ports PE1 and PEn and the output ports PS1 and PSn and afew of the cross-connections between these ports. Clearly all the inputports PE1 to PEn have substantially the same internal architecture, andlikewise all the output ports PS1 to PSn. All the cross-connectionsbetween these ports may be deduced by simple extrapolation.

Each input port PE has, connected to its optical fiber input FE, ademultiplexer 16 with L output channels 16-1 to 16-L (genericdesignation 16) each presenting separately any modulated signal presenton the input fiber FE. There is on each of these channels 16-1 to 16-L arespective first wavelength converter 18-1 to 18-L (generic designation18) for extracting data presented to it at the output of thedemultiplexer 16 at a specific wavelength λ1 to λL. The converters aregenerally based on semiconductor optical amplifiers (SOA) and thereforealso have amplification and even regeneration properties.

The output from each first converter 18 is presented to a respectiveinput of a first multiplexer 20 with L inputs for grouping all theoutputs of the converters 18-1 to 18-L onto the same output channel 22.The first n multiplexers 20 corresponding to all n input ports PE1 toPEn have the same number of respective output channels 22-1 to 22-n(generic designation 22). Thus for each input port PE1 to PEn, theoutput of the system described presents the signals present on the inputfibers FE1 to Fen, after conversion, on internal carrier wavelengths (λ1to λL). For each input port, the combination of the demultiplexer 16,the converter 18 and the first multiplexer 20 forms an input conversionand amplification stage.

The output 22 of each first multiplexer 20, which constitutes a singlechannel, is presented to a respective input of a number K of opticaldelay lines 24-1 to 24-K (generic designation 24) of the time-delay unit14. In the diagram, the configuration provides K time-delay linesspecific to each output 22 of the first multiplexer, although inpractice these lines may be shared if the wavelengths coming from thevarious input multiplexers are different. Each of the delay lines of aset 24-1 to 24-K imposes a specific time-delay. In the present example,the range of time-delays begins with a null value (zero time-delay),taking the form of a direct connection to the line 24-1. Therelationship between the time-delays typically follows a linearprogression; for example, the shortest non-zero time-delay imposed (line24-2) has a value τ and the subsequent lines of the progressionrespectively impose time-delays of τ, 2τ, 3τ, etc. The value τ isgenerally equal to the fixed size (temporal duration) of a packet.

The k outputs of the delay lines of each input port constitute bufferoutputs enabling time division and in particular concatenation of dataarriving simultaneously.

Each output port PS1 to PSn comprises L groups of inputs 26-1 to 26-L(generic designation 26) each having a number n*K of individual inputsconnected to receive a respective output of the K delay lines for eachof the n input ports PE. Each group of n*K inputs 26-1 to 26-L isreceived by a first selection unit 27-1 to 27-L for selecting one inputport PE from the n input ports with a time-delay k from 1 to K. The dataselected by these selection units 27-1 to 27-L is then broadcast by arespective star optical coupler system 28-1 to 28-L (generic designation28) with L outputs. Each of the L outputs of each set is connected to asecond selection unit 29 with L inputs and L outputs. These L outputsare connected to a respective second multiplexer 30-1 to 30-L (genericdesignation 30).

FIG. 4 shows in more detail a system comprising a first selection unit27, a star optical coupler system 28, and a second selection unit 29.

The first selection unit 27 comprises a respective SOA specific to eachof the n*K inputs, in other words a total of n*K input SOAs (SOA-E),each of which is used as an optical gate for selecting an input line. Inservice, one selected SOA-E is turned on (switch closed) to launch thepassed data, and the n*K−1 other SOA-Es are turned off (switch open) toblock the data on their respective channels.

Similarly, the second selection unit 29 comprises a respective SOAspecific to each of the L outputs, in other words a total of L outputSOAs (SOA-S). Each of these is used as an optical gate PO for selectinga wavelength in association with the input of the multiplexer 30. Inservice, one selected SOA-S is turned on (switch closed) to launch thepassed data, and the other L−1 SOA-Ss are turned off (switch open) toblock the data on their respective channels.

The association of an SOA-S i of the selection unit 29 with the input iof the multiplexer 30 (where i is a number from 1 to L) selects thelength i from all the data selected by the first selection unit. Amultiplexer of this kind is provided for each group of inputs 26-1 to26-L.

Selection by the FIG. 4 system proceeds in two stages, based on the factthat each input line receives all the data (L wavelengths) coming froman input port (1 to n) and having a time-delay (1 to K), yielding n*Kinput lines for each output line (one wavelength at one output port):

-   -   a first step in which only one of the n*K inputs is selected by        the SOA-E of the first selection unit 27 at a given time, by        activating the corresponding SOA-E (switch closed); this unit 27        therefore constitutes a first selection stage; the set of data        (L wavelengths) coming from the input port PEj with the        time-delay pK is then broadcast by the n*K:L star coupler 28 to        the L SOA-Es of the second selection unit 29; and    -   a second step for selecting the wavelength λi at the level of        the second selection unit 29, which constitutes the second        selection stage; the SOA-S corresponding to the wavelength λi is        activated (switch closed) so that all of the data (L        wavelengths) is sent to port i of the multiplexer; there is        therefore obtained at the output only data coming from the        wavelength λi of the input port PEj that was subjected in the        matrix to the time-delay pK.

The respective output 32-1 to 32-L of each of the L second multiplexers30 is connected to a respective second wavelength converter 34-1 to 34-L(generic designation 34) before reaching the output fiber FS of theport. These second converters 34 convert the data that is presented totheir input from any of the L different wavelengths λ1 to λL to thewavelengths associated with each group. Note that these secondconverters also serve as regenerators; in addition to their wavelengthconversion function, they regenerate optical packets at each switchingmatrix output in order to restore optical signal quality and to enablethe cascading of a plurality of switch matrices.

The output of each of the L second converters 34 is presented to arespective input of a third multiplexer 36 with L input channels and oneoutput channel connected to the output fiber FS of the port.Accordingly, any output port can send on its output fiber FS any datareceived at the various inputs of the input ports PE1 to PEn at any ofthe wavelengths λ1 to λL, with any contention resolved by adaptation ofthe carrier wavelength (this is called spectral resolution) or byimposition of time-delays (this is called temporal resolution).

This provides two solutions to the contention problem, either byprocessing in the spectral domain (by wavelength conversion) or byprocessing in the time domain (by imposing time-delays).

Below, the term “internal port” refers to any internal input or outputof the switching matrix.

With this kind of architecture, the number of internal ports comprisingsemiconductor optical amplifiers SOA-Es (first selection unit 27) andSOA-Ss (second selection unit 29) is very large, even in wavelengthdivision multiplex mode operation. This is because it is necessary toprovide nK+L internal ports for each output wavelength, which implies intotal nL*(nK+L) semiconductor optical amplifiers. That conventionalsolution therefore implies what is known as complete conversion. Morethan this, because of considerable optical losses in the opticaldistributors of the matrix and the cascaded semiconductor opticalamplifiers, the optical signals from the matrix are degraded and requirethe presence of regeneration interfaces, namely the above-mentioneddemultiplexer 16, converter 18 and multiplexer 20.

In view of the above, the invention proposes simplified nodearchitectures based on only partial use of wavelengthconversion/regeneration in order to render the hardware volumeconstraints of the optical switching matrices more flexible.

The Applicant has discovered that adequate management of even large datastreams can be achieved with correct resolution of potential contentionwithout it being necessary to provide complete wavelength conversioncapacity at all of the ports, as required by conventional switchingmatrix architectures, of which FIG. 3 represents one example.

To be more precise, a first aspect of the invention proposes a systemfor switching optical signals with carrier wavelength conversioncapacity, comprising a set of input ports, a set of output portsfunctionally connected to the input ports so that an input signalpresented to one of the input ports can be selectively routed to atleast one of the output ports, and wavelength conversion means providinga capacity for converting an input signal carrier wavelength to at leastone other output port output wavelength,

-   -   characterized in that the wavelength conversion capacity of the        conversion means is limited by at least one of the following        three limitation means i) to iii):    -   i) for at least one of the output ports, no wavelength        conversion may be applied for sending a signal from an input        port;    -   ii) for at least one of the output ports, wavelength conversion        may be applied for sending a signal from an input port, but to        only a restricted number of wavelength values from the number L        of different wavelength values accepted at the input, this        restricted number being greater than 0 and less than L, and    -   iii) for only a restricted number of output ports less than the        total number of output ports of the switching system, wavelength        conversion may be applied for sending a signal from an input        port to any wavelength value from the number L of different        wavelength values accepted at the input.

In a fist variant, the wavelength conversion capacity of the conversionmeans is limited so that wavelength conversion may be applied forsending a signal from an input port via any output port, but only, ateach of the output ports, to a restricted number x1 of wavelength valuesfrom the number L of different wavelength values accepted at the input,x1 being greater than 0 and less than L.

In a second variant, the wavelength conversion capacity of theconversion means is limited so that wavelength conversion may be appliedfor sending a signal from an input port to any wavelength value from thenumber L of different wavelength values accepted at the input but onlyfor switching to a restricted number x2 of output ports, x2 beinggreater than 0 and less than the number of output ports of the system.

The system may be adapted to switch signals presented in the form ofoptical data packets.

In the first variant, the limitation of the capacity of the conversionmeans is applied at the level of at least one of the output ports, eachoutput port at which the limitation is applied comprising a first numberof signal line inputs from input ports and a second number L of outputlines, this second number representing the number of differentwavelengths at the output ports, and at least one of the output lines ofthis second number of lines has no wavelength conversion means, servingonly to send at the output a signal with the same wavelength as that atwhich the signal is received at the input.

Each output port at which a wavelength conversion limitation is appliedmay comprise a simplified selection unit for grouping onto each outputline, without conversion, signals coming from input lines having thesame wavelength as the output line, the unit further comprising, foreach output line without conversion, spatial selector means forselecting input lines, the selector means having no spectral selectionmeans and being coupled at their output by coupling means to the outputline corresponding to the wavelength of the spatial selector unit.

It may further comprise temporal selection means for delaying a signalfrom an input port before it is sent at the output of an output port,the temporal selection means presenting to the output ports a number Kof copies of signals received at the input ports, each copy beingtime-shifted relative to the others.

Each output port at which a limitation of wavelength conversion isapplied may comprise a set of input lines leading to output linesprovided with no conversion means, the set comprising, for a number n ofinput ports, a number n*K of lines, one for each of the K time-shiftedcopies coming from each of the n input ports.

The n*K lines of the set may be presented to the input of the simplifiedselection unit, which produces at its output a number (L-x1) of outputlines equal to the total number L of different wavelength valuesaccepted at the input of the system less the restricted number x1 ofwavelength values for which carrier wavelength conversion is provided.

Each output port may then comprise a number x1 of sets of input lineseach leading to a respective one of x1 output lines with wavelengthconversion and each comprising n*K input lines, the output portcomprising n*K(x1+1) input lines, each of the x1 sets of lines furthercomprising, for each of the wavelength values of the restricted numberx1 of wavelength values for which carrier wavelength conversion isprovided:

-   -   a spatial and temporal selection stage receiving at its input a        number n*K of input lines, one for each of the K time-shifted        copies coming from each of the n input ports, and, using a nK:L        coupler, producing a number L of outputs equal to the total        number L of wavelength values accepted at the input of the        system,    -   a wavelength selection system comprising a spatial selection        stage associated with a multiplexer receiving the L outputs at        its input and selectively producing one of them at its output,        and    -   wavelength conversion means receiving at their input the output        of the multiplexer and connected at their output to an output        line.

The system may further comprise a multiplexer with L inputs eachreceiving a respective one of the (L-x1) output lines of the simplifiedselection units and the x1 outputs of the set of multiplexers and anoutput sending on an output fiber of the corresponding output port.

It may have a number n of input ports and a number n′ of output ports,the numbers n and n′ being equal or different, each input portcomprising a spectral multiplex comprising a number L of carriers havingL respective wavelengths, the system further comprising:

-   -   a first buffer stage for imposing a number K of mutually        time-shifted copies of each of the n optical input signals,    -   a second stage for converting each of the n*K multiplexes from        the first stage into a number of copies equal to n′*(x1+1), and    -   a third selection stage for selecting L optical signals from the        nK(x1+1) multiplexes received by an output port PS.

The n*K lines of the set may be presented to the input of the simplifiedselection unit, which produces at its output a number of output linesequal to the total number L of wavelength values accepted at the inputof the system.

It may further comprise a multiplexer with L inputs each receiving arespective one of the output lines of the simplified selection units andan output to an output fiber of the corresponding output port.

The second variant of the system may have a number n of input ports anda number n′ of output ports, the numbers n and n′ being equal ordifferent, each input port comprising a spectral multiplex comprising anumber L of carriers having L respective wavelengths, and the systemfurther comprising:

-   -   a first buffer stage for imposing a number K of mutually        time-shifted copies of each of the n optical input signals,    -   a second stage for converting each of the n*K signals from the        first stage into a number of copies equal to L*x2+n′−x2, and    -   a third selection stage for selecting L optical signals from the        n*KL multiplexes received by an output port PS with total        wavelength conversion and for selecting L optical signals from        the n*K multiplexes received by an output port without        wavelength conversion.

A second aspect of the invention provides an optical communicationsnetwork comprising at least one node for connecting input and outputlines, characterized in that the node comprises at least one switchingsystem according to the first aspect of the invention connected to a setof input lines at its input ports and to a set of output lines at itsoutput ports.

The switching system may then be further connected to at least onegateway.

The network may manage contention by temporal distribution of packets,in particular if the packets in contention may not be subjected towavelength conversion because of the limitation of wavelength conversioncapacity, and by spectral and temporal distribution of packets, inparticular if the packets in contention may be subjected to wavelengthconversion.

A third aspect of the invention concerns the use of a switching systemaccording to the first aspect of the invention for switching datastreams in a communications network node with management of contentionby temporal distribution of the data streams, in particular if the datastreams in contention may not be subjected to wavelength conversionbecause of the limitation of wavelength conversion capacity, and byspectral and temporal distribution of the data streams, in particular ifthe streams in contention may be subjected to wavelength conversion.

A fourth aspect of the invention concerns a method of switching opticalsignals with a carrier wavelength conversion capacity, comprising a setof input ports, a set of output ports functionally connected to theinput ports so that an input signal presented to one of the input portsmay be selectively routed to at least one of the output ports,wavelength conversion means for providing a capacity for converting aninput signal carrier wavelength to at least one other output wavelengthat the output of an output port,

-   -   characterized in that the wavelength conversion capacity is        limited by using at least one of the following three limitation        possibilities i) to iii):    -   i) for at least one of the output ports, no wavelength        conversion may be applied for sending a signal from an input        port;    -   ii) for at least one of the output ports, wavelength conversion        may be applied for sending a signal from an input port, but to        only a restricted number of wavelength values from the number L        of different wavelength values accepted at the input, this        restricted number being greater than 0 and less than L, and    -   iii) for only a restricted number of output ports less than the        total number of output ports of the switching system, wavelength        conversion may be applied for sending a signal from an input        port to any wavelength value from the number L of different        wavelength values accepted at the input.

The optional aspects and variants described in the context of the systemor the network conforming to the above-mentioned first and third aspectsof the invention apply mutatis mutandis to this method.

The invention and the advantages stemming from it will become moreclearly apparent on reading the following description of preferredembodiments, which are provided by way of non-limiting example only,which description is given with reference to the appended drawings, inwhich:

FIG. 1, already described, is a simplified diagram of an opticalcommunications network in which the invention may be used;

FIG. 2, already described, is a simplified general block diagram of anoptical switching matrix, showing in particular the relations betweenthe input and output port units;

FIG. 3, already described, is a more detailed block diagram of anoptical switching matrix, showing internal elements of the architectureof the input and output port units and buffers employing delay lines;

FIG. 4, already described, is a detailed diagram showing SOA in twosuccessive selection stages at the output of the FIG. 3 optical matrix;

FIG. 5 a is a simplified diagram of the architecture of an opticalswitching matrix conforming to a first variant of the invention;

FIG. 5 b shows part of FIG. 5 a to a larger scale, showing the outputsof an output port of the matrix in more detail;

FIG. 6 a is a simplified diagram of the architecture of an opticalswitching matrix conforming to a second variant of the invention;

FIG. 6 b shows part of FIG. 6 a to a larger scale, showing the outputsof an output port of the matrix in more detail;

FIG. 7 is a more detailed diagram of an output port architecture andcertain internal connections conforming to the first variant of theinvention;

FIG. 8 is a more detailed diagram of an output port architecture andcertain internal connections conforming to the second variant of theinvention;

FIG. 9 is a diagram showing the use of an optical switching matrix ofthe invention in a network with a connection to a gateway, depictingoperation with data streams leaving the gateway; and

FIG. 10 is a diagram showing the use of an optical switching matrix ofthe invention in a network with a connection to a gateway, depictingoperation with data streams arriving at the gateway.

The embodiments of the invention described relate to switching opticaldata in the form of packets, although other data structures may beenvisaged.

As explained above, the invention stems from the applicant's observationthat, depending on the network architecture concerned,conversion/regeneration of all wavelengths and/or on each input oroutput optical path is not always necessary in optical switchingmatrices, for example those which manage optical packets. Thus,according to the invention, to simplify the architecture of the nodes,the use of these converters/regenerators is limited, for example byequipping only certain ports or only part of each port. In this way,since complete wavelength conversion (i.e. conversion of all availablecarrier wavelengths at all output port outputs) is not provided at theoutput of the optical switching matrices for the ports not provided witha converter/regenerator, the only input wavelengths in contention for agiven output wavelength λi are the wavelengths with the same value λi ineach input fiber.

Thus, at a given output port, the second level of wavelength selectionperformed by the optical cross-connection system 29 with converters 34in the architecture of FIGS. 3 and 4 may be dispensed with, or at leastgreatly simplified. This is because systematic wavelength selection isthen no longer necessary and the size of the broadcasting opticalcouplers (1:nL couplers) may be reduced.

Hereinafter, L is the number of available carrier wavelengths forsignals processed by the switching matrix and n is the number of inputor output ports of the switching matrix.

Two variants of the invention are envisaged:

-   -   variant 1: all ports have only a certain number of wavelengths        for which conversion is provided, which constitutes the solution        referred to hereinafter by the expression “x1/L wavelength λ        conversion possibilities at all ports”, where x1 is an integer        greater than 0 and less than L, and    -   variant 2: certain output ports have no conversion capacity and        others have provision for complete conversion, which constitutes        the solution referred to hereinafter by the expression “x2/n        ports with complete wavelength λ conversion”, where x2 is an        integer greater than 0 and less than n.

In the present context, the expression “complete conversion” means thefacility to convert any input wavelength of a set of wavelengths λ1 toλL to any other wavelength of the set. On the other hand, a conversionpossibility reduced to a fraction x1/L or x2/n of these possibilitiesrespectively means that only the number x1 of the L wavelengths may beconverted, for a given port, or that only the number x2 of the n outputports are provided with means for complete conversion to any of thewavelengths λ1 to λL.

FIGS. 5 a and 5 b represent the generic structure of variant 1. Thesefigures represent in simplified form an n×n switching matrix 10 as shownin FIGS. 3 and 4 with its connections to input fibers FE and outputfibers FS. The n input ports PE1 to PEn may have a structure that isanalogous to that of FIG. 3 and for conciseness is not described again.However, according to one advantageous aspect of the invention, theconversion/regeneration stage of the input ports may be dispensed with,consisting of the following combination: first demultiplexer 16, firstwavelength converter 18, first multiplexer 20. This is because thesimplified architecture of the invention limits losses and may renderthis stage unnecessary. The following descriptions of the figures arebased on this option with no conversion/regeneration at the input ports.In this case, each of the input fibers FE1 to Fen drives an opticalcoupler (not shown) that feeds the respective K inputs of the delaylines 24-1 to 24-K; the fibers FE1 to FEn are then connected directly tothe points 22-1 to 22-n in FIG. 3.

Differing in this respect from the architecture of FIGS. 3 and 4, theset of n output ports PS is simplified by providing only a number x1 ofthe L port outputs 32-1 to 32-L adapted to be connected to a respectiveconversion/regeneration stage 34-1 to 34-x 1, as shown more precisely inthe view of the outputs to a larger scale shown in FIG. 5 b. For each ofthese x1 outputs, the upstream optical paths, namely the multiplexer 30,the optical coupler 28 and the input group 26, are in practice analogousto those of FIG. 3.

The output (broadcast) stage is simplified at the level of the L-x1other outputs 32-x 1+1 to 32L. These drive the lead multiplexer 36 ofthe output fiber FS directly, with no interposed conversion/regenerationstage 34. Within the output port, this simplifies optical coupling inparticular, subject to the addition of demultiplexers, as explainedbelow. If this architecture is compared to a conventional broadcast andselect structure, for example as shown in FIG. 3, it is found that atthe broadcast—cross-connection 12—level the size of the optical couplersdownstream of the delay lines may be reduced from n*L to n*(x1+1), wherex1 is the number of wavelengths with the conversion facility from the nwavelengths. This is because it is sufficient in this instance tobroadcast the data to the n output ports PS-1 to PS-n (the factor n iscommon to the two formulas) and then per output port: one broadcast toeach of the x1 groups with conversion plus one broadcast to all the L-x1groups without conversion. The total number of broadcasts is thuslimited to n*(x1+1). This is mainly because, for each of the groupswithout conversion (for each of the output port wavelengths withoutconversion), contention is resolved only between data coming from eachof the input ports on the same wavelength, whereas for groups withconversion it is necessary to resolve contention between data arrivingon all the wavelengths (λ1 to λL) of all the input ports. In both cases,temporal flexibility for resolving contention is retained (access to theK time-delays for all the wavelengths).

Consider by way of example a 2×2 switch with four wavelengths per port.At the output ports, wavelengths λ1 and λ2 are with conversion andwavelengths λ3 and λ4 are without conversion. For the groups 1 and 2, itis possible to receive packets at λ1, λ2, λ3 and λ4 at one of the twoinput ports, whereas for the ports 3 and 4 it is possible to receiveonly the packets at λ3 and λ4 at one of the two input ports,respectively. The number of broadcasts after the time-delays istherefore 6 instead of 8 in the case of complete conversion.

FIGS. 6 a and 6 b, which are diagrams analogous to those of FIGS. 5 aand 5 b, represent the generic structure of variant 2.

In this situation the reduction of the conversion capacities by thefactor x2/n is obtained by providing only a number x2 of the n outputports PS1 to PSn with full conversion and regeneration capacities. Eachof these x2 ports is therefore in all respects identical to an outputport PS from FIG. 3, the n output ports 32-1 to 32 n driving themultiplexer 36 of the output fiber FS via L respectiveconversion/regeneration stages 34-1 to 34-L.

In this situation, the simplification results from providing no outputconversion and regeneration stage for the n−x2 other output ports PSx2+1to PSn. As shown more precisely by the view of the outputs to a largerscale in FIG. 6 b, the L outputs 34-1 to 34-L of each of these n−x2other ports drive the lead multiplexer 36 of its output fiber FSdirectly. In these n−x2 ports, this simplifies the optical couplings inparticular and eliminates the second multiplexers 28, subject to theaddition of demultiplexers/distributors, as explained below, as forvariant 1.

Thus in both variants 1 and 2, the complexity of the optical switchingmatrix is greatly reduced, compared to the prior art, as there is onlypartial broadcasting within the switching matrix.

FIG. 7 shows in more detail the internal architecture of a switchingmatrix 40 conforming to the above-mentioned variant 1, the generalstructure of which has been described with reference to FIGS. 5 a and 5b. All of the n partial conversion output ports PSCP are identical, andonly the port PSCP1 that drives the output fiber FS1 is represented indetail. Likewise, all the n input ports are identical, and only theoutputs 1 to k of the delay lines coming from the two input ports PE1and PEs are shown (cf. FIG. 3).

Total wavelength conversion with regeneration is provided by the firstx1 input groups 26-1 to 26-x 1, of which only the first and the x1 ^(th)are shown. The optical processing means and paths for each of thesegroups 26-1 to 26-x 1 are the same as for any other input group 26 fromFIG. 3. Thus for each there are, in succession, a coupling system nK:L28, a second multiplexer 30, and a conversion/regeneration stage 34. Theoutput of each of these x1 stages 34-1 to 34-x 1 drives the lead outputmultiplexer 36 of the output fiber FS.

The L-x1 other inputs of this output multiplexer 36 come directly froman optical selection structure 42 that gathers each of n*K delay lineoutputs at a respective demultiplexer 44. To be more precise, theoptical selection structure 42 receives at its n*K multiplexers 44 the Koutputs of the delay lines 24-1 to 24-4 for each of the n input portsPE1 to PEn.

The L-x1 outputs of these n*K demultiplexers 44, corresponding to theL-x1 wavelengths at the output ports without conversion, are gathered bya set of selection strips made up of a number d of selection strips 46-1to 46-d, where d is equal to L-x1. Each strip receives the data carriedby the same wavelength but coming from n*K demultiplexers 44. Thus eachof the d strips outputs to a respective nK:1 coupler 48-1 to 48-d(generic designation 48) any of the inputs presented to an input portfiber FE1 to FEn with any of the available K time-delays of the delaylines 24-1 to 24-K, but only at the original wavelength present at theinput fiber. Each output of the couplers 48 is presented to therespective input of the output multiplexer 36 corresponding to thewavelength at the output of each selection unit 46. Note that theselection unit therefore has L inputs, of which a number x1 are withwavelength conversion and the remaining L-x1=d are without wavelengthconversion.

FIG. 8 shows in more detail the internal architecture of a switchingmatrix 50 conforming to the above-mentioned variant 2, the generalstructure of which is described with reference to FIGS. 6 a and 6 b. Theoutput ports are of two types: with complete conversion (genericdesignation PSCC) and without conversion (generic designation PSSC).There are x2 output ports with complete conversion PSCC1 to PSCCx2.Their operation and their connections to the n input ports PE1 to PEnvia the delay lines 24-1 to 24-n conform to the description given withreference to FIG. 3 and for conciseness are not described again. Theoutput fibers FS1 to FSx2 of these PSCC ports therefore enable each ofthem to forward all data presented to the input ports PE1 to PEn on anyof the L wavelengths, with the facility of selecting from the Ktime-delays.

On the other hand, the n-x2 other output ports PSSCx2+1 to PSSCn withoutconversion provide no conversion of the wavelengths of data presented totheir inputs. Their function is limited to selecting on their respectiveoutput fiber FS all data presented to the input ports PE1 to PEn, withthe facility of selecting from the K time-delays to resolve contentionwavelength by wavelength.

To this end, each output port PSSC without conversion comprises aselection system 52 with n*K inputs, one for each of the respective ninput ports PE1 to Pen and each of the K available time-delays. Theselection system 52 comprises a number n*K of demultiplexers 54, one foreach input, and a number L of selection switches 56, each associatedwith a coupler 58, in the manner of the FIG. 7 selection system 42. Theoutput from each of the n*K demultiplexers 54 is sent to each of the Lselection strips 56, each of these strips processing only onewavelength. The L couplers 58 drive directly L respective inputs of theoutput multiplexer 36, associating the output wavelengths of theselection unit 56 one by one with the input wavelengths of themultiplexer 36, with the result that said multiplexer is able to outputone of the n*K inputs on its associated output fiber.

The architectures of the above-mentioned variants 1 and 2 arefunctionally equivalent, and considerably simplify the prior artarchitectures (cf. FIG. 3) by reducing the number of input wavelengthconverters; the input fibers are advantageously connected directly tothe optical buffer memory consisting of the delay lines 24-1 to 24-K(with no buffers, k=1), and each comb of wavelengths is thereforebroadcast to each delay line. Also, the size of the optical coupler tothe output stage is reduced compared to that in FIG. 3. This is becauseeach time-delay is broadcast to each output port PS, but not necessarilyat each wavelength of the output ports. There is total broadcasting onlyat the output wavelengths with conversion, but limited broadcasting tothe ports without conversion is sufficient, the remainder being achievedby spectral demultiplexing and one-to-one correspondences between inputwavelengths and output wavelengths. Thus the selection process issimilar to that of FIG. 3 for ports with wavelengthconversion/regeneration (in this instance, two stages of optical gates),but it is simplified for ports without wavelength conversion,necessitating only one stage of optical gates (there is no additionalactive wavelength selection stage, wavelength selection being achievedpassively by means of the demultiplexer 44 in the former case or thedemultiplexer 54 in the latter case).

It is clear from FIGS. 7 and 8 that the above-mentioned value x1 or x2is to be understood as a factor that represents the rate of partial useof conversion/regeneration in the architecture (x1 of the L wavelengthson each output fiber in FIG. 5 a, 5 b or 7 and x2 of the n output fibersin Figure Ga, 6 b or 8). The broadcasting couplers are therefore reducedto one 1:n(x+1) coupler in FIG. 7 and one (Lx+n−x) coupler in FIG. 8.

If x=0, the optical switching matrix offers no wavelength conversion andif x1=L in FIG. 7 or x2=n in FIG. 8 the optical switching matrix has thefull conversion capacity.

In the embodiments shown in FIGS. 7 and 8, which employ optical gates inthe form of semiconductor optical amplifiers employing wavelengthdivision multiplexing, to limit the number of wavelengths in eachamplifier the value of L should preferably be less than 42. Otherwise,the architecture may be modified as explained in French PatentApplication FR 0015889, using band demultiplexing at the input (b bandsbeing intended to have L/b wavelengths per amplifier).

Using only partial wavelength conversion in accordance with theinvention simplifies the selection stage of the optical data switches.This impacts directly on the volume of hardware necessary forimplementing the optical switching matrices and therefore reduces costs.To illustrate the benefits that may be obtained from the invention,Table I summarizes the volume of hardware used in each case. TABLE IVolume of hardware required for different optical switching matrixconfigurations. Architecture Mux Couplers Optical gates λ conversionStandard Mux L λs 1: K (n) [nK + L] *nL 2nL (cf. FIG. 3) ([L + 3] *n) 1:nL (nK) and L < 32 NK: L (nL) Variant 1: Without PE: 1: K (nb) [nK + x1]*nL Nx1 without x1/n λ conv. Mux L 1: n [x + 1] (nk) i.e. PE poss./allλs ([x1 + 1] *n) nK: L (nx1) [nK + L] *nx1 n (L + x1) ports With PE: nK:1 (n* [L − x]) (λ with with PE (FIGS. 4a, 4b, 6) Mux L conv.) + λs ([x1 + 3] *n) nK* [1 − x1) *n In both (λ without cases: conversion) Mux L− x1 (nK* [L − x1] *n Variant 2: Without PE: 1: K (n) [n²K + Lx2] *L Lx2without X2/n ports Mux L 1: [Lx + n − i.e. PE with complete λs ([L + 1]*x2 + x2] (nK) [nK + L] *Lx2 (nL + Lx2) λ conv. nK* [n − x2]) nK: L(x2L) (ports with with PE (FIGS. 5a, 5b, 7) With PE: nK: 1 ([n − x2] *Lconversion) + Mux L nKL* (n − x2] λs([L + 1] *x2 + (ports nK* [n − x2] +2n without conversion)

The reduction in terms of semiconductor optical amplifiers compared to aconventional architecture such as that shown in FIG. 1 is (L−x1)*nL forvariant 1 (FIGS. 5 and 7) and (n−x2)*L² for variant 2 (FIGS. 6 and 8).For example, a 4×4 architecture may be envisaged with 16 wavelengths perfiber output and an 8-position buffer (n=4, L=16, K=8).

By restricting wavelength conversion to half of the resources (50% ofthe wavelengths on each fiber in the case of variant 1 (FIGS. 6 and 8)and 50% of the fibers in the case of variant 2 (FIGS. 7 and 9)), 512semiconductor optical amplifiers are saved in both cases, which isalmost 20% of the total number of amplifiers used in a FIG. 3 prior artarchitecture of similar capacity. For example, the broadcast couplers inFIG. 8 may then be 1:36 couplers, instead of the 1:64 couplers in FIG.3.

In practice, a 1:64 coupler will have to be used in this example, but inother cases the reduction in coupling could be a good match to existingcoupler sizes (taking seven wavelengths with wavelength conversion fromthe 16 wavelengths of the same example in FIG. 8 would result in a 1:32coupler).

Clearly it is equally possible to combine the above-mentioned variants 1and 2, for example to produce a switching matrix in which at least oneoutput port provides conversion of all wavelengths while at least oneother port is restricted to converting only a restricted number ofdifferent wavelengths processed at the input ports, or a switchingmatrix with certain ports offering partial conversion and othersoffering no conversion.

FIGS. 9 and 10 show examples of the operation of an optical switchingmatrix for switching optical packets conforming to either of variants 1and 2 of the invention for routing data packets to a node of an opticalnetwork including a local connection gateway.

In the example of FIGS. 9 and 10, a switching matrix 60, which may takethe form of the matrix 40 of FIG. 7 or the matrix 50 of FIG. 8, isconnected to a gateway 70 and to an optical ring network 2. Moreparticularly, at least one output of the gateway is connected to arespective input port PEp of the matrix 60 and at least one output portPSp of the matrix is connected to a respective input of the gateway.These ports PEp and PSp thus respectively enable the gateway 70 to sendand receive optical data packets on the network by means of aconventional connection. The gateway typically provides a connectionbetween the network and a sub-network, for example a metropolitan areanetwork.

The other input and output ports of the switching matrix 60 areconnected to the network for routing data in transit in the network andfor adding or dropping data on behalf of the gateway.

Communication between the gateway 70 and the network is managed by anMAC protocol. This protocol, which is used in particular when there isno memory along the path, guarantees that the packets from the sourcereach the destination. In this case, the packets remain on a carrier atthe same wavelength throughout their end-to-end path.

However, depending on particular circumstances, it may also be necessaryto apply carrier wavelength conversion at the switching matrix 60between the gateway 70 and the network 2 or to signals in transit in thenetwork via the matrix.

Thus paths via the matrix are provided that may be used withoutwavelength conversion, in addition to paths with wavelength conversionfor dealing with contention. The MAC protocol also handles the choice ofwhether or not to use wavelength conversion at the matrix 60.

Switching with partial conversion in accordance with the invention thenmeets all requirements, at the same time as simplifying thearchitecture.

In the mode of operation shown in FIG. 9, data is sent from the gateway70 to an input port PEp of the switching matrix 60 (arrows F1). Inparallel with this, data traffic in transit crosses the matrix (arrowsF2 and F3). Managing streams across the switching matrix then calls forpartial wavelength conversion for each fiber, with contention resolvedby the MAC protocol. Thus a portion of the traffic is routed withoutwavelength conversion and only traffic in contention is subjected towavelength conversion.

In the mode of operation shown in FIG. 10, the switching matrix 60routes data from the network 2 on respective wavelengths to the gateway70 (arrows F4 and F5), in parallel with data in transit crossing thematrix (arrows F6 and F7). In this case it is not necessary to applywavelength conversion to the data F4 and F5 from the network to transferit to the gateway. Contention is resolved by the MAC protocol for thering forming the network, and there is no contention between the matrixand the gateway (if there are the same number of wavelengths at theinput PE of the matrix connected to the ring as at the output port PSp).If a terminal connected to the gateway is able to access the matrix at agiven wavelength, the matrix always remains available for communicationbetween the matrix 60 and the gateway 70, since there is no contentionwith traffic coming from PEp.

Numerous embodiments and variants that do not depart from the scope ofthe invention may be envisaged, for example with regard to the hardware,functional, and management means and the dimensions of the hardware.

1-23. (canceled)
 24. System for switching optical signals with carrierwavelength conversion capacity, comprising a set of input ports(PE1-PEn), a set of output ports (PS1-PSn) functionally connected to theinput ports so that an input signal presented to one of the input portsmay be selectively routed to at least one of the output ports, andwavelength conversion means (34) providing a capacity for converting aninput signal carrier wavelength to at least one other output port outputwavelength, said wavelength conversion capacity of said conversion means(34) being limited by at least one of the following three limitationmeans i) to iii): i) for at least one of said output ports (PS), nowavelength conversion may be applied for sending a signal from an inputport; ii) for at least one of said output ports (PS), wavelengthconversion may be applied for sending a signal from an input port (PE),but to only a restricted number of wavelength values from the number Lof different wavelength values accepted at the input, this restrictednumber being greater than 0 and less than L, and iii) for only arestricted number of output ports (PS) less than the total number ofoutput ports of the switching system, wavelength conversion may beapplied for sending a signal from an input port (PE) to any wavelengthvalue from the number L of different wavelength values accepted at theinput, the system having a number n of input ports (PE) and a number n′of output ports (PS), the numbers n and n′ being equal or different,each input port comprising a spectral multiplex comprising a number L ofcarriers having L respective wavelengths; characterized in that thesystem further comprises: a first buffer stage (24) for imposing anumber K of mutually time-shifted copies of each of the n optical inputsignals, a second stage for converting each of the n*K multiplexes fromthe first stage into a number of copies less than n′, and a thirdselection stage for selecting L optical signals from the multiplexesprovided by the second stage to an output port (PS).
 25. Systemaccording to claim 24, characterized in that said wavelength conversioncapacity of said conversion means (34) is limited so that wavelengthconversion may be applied for sending a signal from an input port (PE1to PEn) via any output port (PS1 to PSn), but only, at each of theoutput ports, to a restricted number x1 of wavelength values from thenumber L of different wavelength values accepted at the input, x1 beinggreater than 0 and less than L.
 26. System according to claim 24,characterized in that said wavelength conversion capacity of saidconversion means (34) is limited so that wavelength conversion may beapplied for sending a signal from an input port (PE1 to PEn) to anywavelength value from the number L of different wavelength valuesaccepted at the input but only for switching to a restricted number x2of output ports (PS1 to PSx2), x2 being greater than 0 and less than thenumber of output ports of the system.
 27. System according to claim 24,characterized in that it is adapted to switch signals presenting in theform of optical data packets.
 28. System according to any of claim 24,characterized in that said limitation of the capacity of the conversionmeans (34) is applied at the level of at least one of the output ports(PS), each output port at which said limitation is applied comprising afirst number of signal line inputs from input ports (PE) and a secondnumber L of output lines, this second number representing the number ofdifferent wavelengths at the output ports (PS), and at least one of theoutput lines of this second number of lines has no wavelength conversionmeans, serving only to send at the output a signal with the samewavelength as that at which the signal is received at the input. 29.System according to claim 24, characterized in that each output port(PS) at which a wavelength conversion limitation is applied comprises asimplified selection unit (42; 52) for grouping onto each output line,without conversion, signals coming from input lines having the samewavelength as the output line, the unit further comprising, for eachoutput line without conversion, spatial selector means (46; 56) forselecting input lines, said selector means having no spectral selectionmeans and being coupled at their output by coupling means (48, 58) tosaid output line corresponding to the wavelength of the spatial selectorunit.
 30. System according to claim 24, characterized in that it furthercomprises temporal selection means (24) for delaying a signal from aninput port (PE) before it is sent at the output of an output port (PS),the temporal selection means presenting to the output ports (PS) anumber K of copies of signals received at the input ports (PE), eachcopy being time-shifted relative to the others.
 31. System according toclaim 24, characterized in that each output port (PS) at which alimitation of wavelength conversion is applied comprises a set of inputlines leading to output lines provided with no conversion means, the setcomprising, for a number n of input ports (PE1 to PEn), a number n*K oflines, one for each of said K time-shifted copies (24) coming from eachof the n input ports.
 32. System according to claim 6, characterized inthat each output port (PS) at which a limitation of wavelengthconversion is applied comprises a set of input lines leading to outputlines provided with no conversion means, the set comprising, for anumber n of input ports (PE1 to PEn), a number n*K of lines, one foreach of said K time-shifted copies (24) coming from each of the n inputports, and further characterized in that said n*K lines of the set arepresented to the input of said simplified selection unit (42), said unitproducing a number (L-x1) of output lines equal to the total number L ofdifferent wavelength values accepted at the input of the system lesssaid restricted number x1 of wavelength values for which carrierwavelength conversion is provided.
 33. System according to claim 32,characterized in that each output port (PS) comprises a number x1 ofsets of input lines each leading to a respective one of x1 output lineswith wavelength conversion and each comprising n*K input lines, saidoutput port comprising n*K(x1+1) input lines, each of said x1 sets oflines further comprising, for each of the wavelength values of saidrestricted number x1 of wavelength values for which carrier wavelengthconversion is provided: a spatial and temporal selection stage (27)receiving at its input a number n*K of input lines, one for each of saidK time-shifted copies coming from each of the n input ports and, using anK:L coupler (28), producing a number L of outputs equal to the totalnumber L of wavelength values accepted at the input of the system, awavelength selection system comprising a spatial selection stage (29)associated with a multiplexer (30) receiving said L outputs at its inputand selectively producing one of them at its output, and wavelengthconversion means (34) receiving at their input the output of saidmultiplexer (30) and connected at their output to an output line. 34.System according to claim 33, characterized in that it further comprisesa multiplexer (36) with L inputs each receiving a respective one of the(L-x1) output lines of said simplified selection units and the x1outputs of the set of multiplexers (30-1 to 30-x 1) and an outputsending on an output fiber (FS) of the corresponding output port. 35.System according to claim 24, characterized in that: it is adapted toswitch signals presenting in the form of optical data packets; saidsecond stage includes means for converting each of the n*K multiplexesfrom the first stage into a number of copies equal to n′*(x1+1), andsaid third selection stage includes means for selecting L opticalsignals from the nK(x1+1) multiplexes received by an output port PS. 36.A system according to claim 6, characterized in that each output port(PS) at which a limitation of wavelength conversion is applied comprisesa set of input lines leading to output lines provided with no conversionmeans, the set comprising, for a number n of input ports (PE1 to PEn), anumber n*K of lines, one for each of said K time-shifted copies (24)coming from each of the n input ports, and further characterized in thatsaid n*K lines of the set are presented to the input of said simplifiedselection unit (52), said block producing at its output a number (L) ofoutput lines equal to the total number L of wavelength values acceptedat the input of the system.
 37. A system according to claim 36,characterized in that it further comprises a multiplexer (36) with Linputs each receiving a respective one of the (L) output lines of saidsimplified selection units and an output to an output fiber (FS) of thecorresponding output port.
 38. System according to claim 24,characterized in that: said second stage includes means for convertingeach of the n*K signals from the first stage into a number of copiesequal to L*x2+n′−x2, and said third selection stage includes means forselecting L optical signals from the n*KL multiplexes received by anoutput port PS with total wavelength conversion and for selecting Loptical signals from the n*K multiplexes received by an output port (PS)without wavelength conversion.
 39. Optical communications network (2)comprising at least one node (4) for connecting input and output lines,characterized in that said node comprises at least one switching system(10; 40; 50; 60) according to claim 24 connected to a set of input linesat its input ports (PE) and to a set of output lines at its output ports(PS).
 40. Network according to claim 39, characterized in that theswitching system is further connected to at least one gateway (70). 41.Network according to claim 39, characterized in that it managescontention by temporal distribution of packets, in particular if thepackets in contention may not be subjected to wavelength conversionbecause of said limitation of wavelength conversion capacity, and byspectral and temporal distribution of packets, in particular if thepackets in contention may be subjected to wavelength conversion.
 42. Useof a switching system (10) according to claim 24 for switching datastreams in a communications network node with management of contentionby temporal distribution of the data streams, in particular if the datastreams in contention may not be subjected to wavelength conversionbecause of said limitation of wavelength conversion capacity, and byspectral and temporal distribution of the data streams, in particular ifthe streams in contention may be subjected to wavelength conversion. 43.Method of switching optical signals with a carrier wavelength conversioncapacity, comprising a set of input ports (PE1-PEn), a set of outputports (PS1-PSn) functionally connected to the input ports so that aninput signal presented to one of the input ports may be selectivelyrouted to at least one of the output ports, wavelength conversion means(34) for providing a capacity for converting an input signal carrierwavelength to at least one other output wavelength at the output of anoutput port, characterized in that said wavelength conversion capacityis limited by using at least one of the following three limitationpossibilities i) to iii): i) for at least one of said output ports (PS),no wavelength conversion may be applied for sending a signal from aninput port; ii) for at least one of said output ports (PS), wavelengthconversion may be applied for sending a signal from an input port (PE),but to only a restricted number of wavelength values from the number Lof different wavelength values accepted at the input, this restrictednumber being greater than 0 and less than L, and iii) for only arestricted number of output ports (PS) less than the total number ofoutput ports of the switching system, wavelength conversion may beapplied for sending a signal from an input port (PE) to any wavelengthvalue from the number L of different wavelength values accepted at theinput.
 44. Method according to claim 43, characterized in that saidwavelength conversion capacity is limited so that wavelength conversionmay be applied for sending a signal from an input port (PE1-PEn) via anyoutput port (PS1-PSn) but, for each of the output ports, only to arestricted number x1 of wavelength values from the number L of differentwavelength values accepted at the input, x1 being greater than 0 andless than L.
 45. Method according to claim 43, characterized in thatsaid wavelength conversion capacity is limited so that conversion may beapplied for sending a signal from an input port (PE1-PEn) to only arestricted number x2 of the number n of output ports (PS1 to PSx), x2being greater than 0 and less than the number of output ports, but witha capacity for wavelength conversion to any wavelength value of thenumber L of different wavelengths accepted at the input.